There is considerable evidence now that small, rubble pile asteroids such as Itokawa, Ryugu and Bennu are `granular aggregates', composed entirely of mobile, unconsolidated material in the form of boulders, cobbles, pebbles and perhaps finer grained material, held together by self-gravity and containing no separate, monolithic core [1]}, [2]}, [3]}, [4]}, [5]}. Granular aggregates on Earth are common in river beds and beaches and have been studied extensively in a wide variety of contexts. They do not fit within the usual solid/liquid/gas classification of states of matter but exhibit a range of phenomena unique to themselves, including granular flow or convection [6]}. For example, large boulders, even when denser than their surroundings, rise to the top of shaken piles instead of sinking to their bottoms, as they would in a liquid or gas, a phenomenon known as the `Brazil Nut Effect' [7]}, [8]}.
